my AOI requires two scene of satellite image but when I try to mosaic the images using ERDAS imagine, the color of the edge is seen forming black line after mosaic! so please how can I remove the image edge color?

My approach would be to draw a border shape around your AOI, save it to a shapefile and the use GDAL's crop_to_cutline command to crop the image to those borders.
After performing this operation on both of your sat. images, you should be fine to mosaic them!